The proliferation of computers and the advent of the Internet and the maturing of the World Wide Web (“web”) have significantly increased the use and dependence of email communication. However, the increased reliance on email has exposed the individuals and corporations that use email to threats from, for example, email viruses, spam, phishing attacks, and the like. Commercially available anti-virus software packages are typically employed to combat such attacks.
Email is a highly distributed server application, where an email message typically travels through a set of servers after entering an enterprise, until the message reaches the addressed end user's inbox. With the huge amount of spam and phishing emails that are reaching enterprise gateways, filtering technologies have become important in ensuring the email flow is preserved clean while the filtering is happening as soon as the emails enter the enterprise perimeter. However, some of the filtering techniques require access to resources that are not widely available (e.g. Active Directory information that stores admin policies, end user preferences, etc.). Therefore, even though filtering is a very expensive operation in terms of computing resources, the filtering is going to be deployed at different places (i.e., email servers and clients) in the overall email infrastructure.
The distributed nature of the email “application,” which includes the email servers and clients, coupled with the rather unpredictable nature of the email infrastructure (e.g., some routing servers might be down making the email traffic take different routes) make it difficult to guarantee that an email reaching the client has actually been filtered by the full set of filters as dictated by, for example, the enterprise's security policy. Another difficulty is ensuring that the email, while being routed towards the email client, is not unnecessarily filtered multiple times by the same filter version, thus adding no value or benefit to performance. These difficulties are further compounded when filters from different vendors are deployed within in the email infrastructure, for example, in cases where different email clients connect with different filter versions (e.g., smart phone vs. MICROSOFT OUTLOOK).